Burner control system

ABSTRACT

A burner control system is disclosed including a burner and fuel supply structure controlling the flow of fuel to the burner. Circuitry for controlling operation of the fuel supply structure includes an AC power supply, a voltage doubler arrangement, a relay, and a condition responsive switch for connecting the relay across the voltage doubler. The condition responsive switch is preferably an electronic switch rendered conductive when flame is sensed at the burner. The relay is operatively energized only in response to an applied voltage which is greater than the peak power supply voltage. If any component of the control circuitry short circuits, or otherwise fails, the relay cannot be operatively energized thus preventing the supply of fuel to the burner.

0 United States Patent [151 3,645,645 1 Goodman Feb. 29, 1972 [54]BURNER CONTROL SYSTEM 3,454,839 7/1969 McIntosh ..3l7/15l X [72]Inventor: Kenneth C. Goodman, Columbus, Ohio Primary Examiner canon BDomy Jr [73] Assignee: Ranco Incorporated, Columbus, OhioAttorney-Watts, Hoffmann, Fisher & Heinke [22] Filed: Aug. 6, 1969ABSTRACT 21 A LN 848062 I 1 pp 0 A burner control system is disclosedincluding a burner and fuel supply structure controlling the flow offuel to the burner. [52] US. Cl ..43l/24, 431/78 Circuitry forcontrolling operation of the fuel supply structure [51] Int. Cl ..F23n5/00 i l d an AC power supply, a voltage doubler arrangement, [58] Fleldof Search ..43.l/24, 69, 7], 78-80; a relay, and a condition responsiveSwitch f connecting the 307352-70; 317/1485 B relay across the voltagedoubler. The condition responsive switch is preferably an electronicswitch rendered conductive [56] References Cited when flame is sensed atthe burner. The relay is operatively UNITED STATES PATENTS energizedonly in response to an applied voltage which is v greater than the peakpower supply voltage. If any component 3,291,183 12/1966 Fairley..431/78 X of the control circuitry short circuits, or otherwise fails,the 3,463,600 8/1969 relay cannot be operatively energized thuspreventing the Reichow X upply of fuel to the burner. 3,288,195 11/1966Thompson ..43l/24 3,445,173 5/1969 Malavasi et al. ..43 1/69 X 12Claims, 2 Drawing Figures F. I .L 7. T i 7/ l I 255 L/ 7/b l l l l l?5a. l l 70 l l I l L2 1 4 l l t l L I l m r l i l l l i "l FLAME TE/4Ll Tlj OE I 27a, l 27 /6N/7'/0N L BURNER CONTROL SYSTEM BACKGROUND OF THEINVENTION Field of the Invention A The present invention relates toburner control systems and more particularly relates to burner controlsystems in which fuel flow to a burner is maintained in response tocombustion taking place at the burner.

The Prior Art Burner control systems have been developed for terminatingfuel flow to a burner when combustion at the burner terminates. Inrecent years the development of electronic semiconductor devices hasenabled development of flame supervising circuits for electronicallysensing the presence of flame at burners and controlling fuel valves inresponse to the sensed flame condition.

Generally, such circuits included a fuel valve operating solenoid coil,or a relay coil for controlling operation of a fuel valve. Energizingcircuits for such coils were completed through semiconductor switchingdevices, such as silicon controlled rectifiers (SCR), or theirequivalent.

The conductive condition of the semiconductor elements was controlled byone or more circuits which responded to temperature, physical presenceof a flame at the burner, radiation from a flame, or other conditionswhich indicated that combustion was occuring at the burner.

The design of such circuitry was based upon the premise that so long asthe control circuitry functioned properly and flame was present at theburner, the energizing circuit for the coil was completed through theelectronic switch. If flame detecting circuitry malfunctioned, orcombustion at the burner for some reason terminated, the electronicswitch was rendered nonconductive to interrupt the energizing circuitfor the coil. Deenergization of the coil terminated the supply of fuelto the burner by closing the fuel valve.

Systems of the type referred to were most frequently applied tocombustion equipment employing gas or volatile fuels since continuedsupply of such fuel in the absence of flame at the burner couldeventually result in an explosive accumulation of unburned gas orvolatilized fuel.

' Due to the potentially dangerous situations which could result fromaccumulation of unburned fuel, it was necessary to design controlsystems so that failure or malfunction of any portion of the controlcircuitry did not lead to accumulations of unburned fuel. This type ofdesign is commonly referred to as fail safe."

Fail safe control systems utilizing semiconductor elements werenecessarily designed so that the combustion detection circuitry produceda combustion indicating signal only when combustion occurred at theburner and then only if the combustion detection circuitry wasfunctioning properly. Likewise, the electronic switch for controllingconduction in the coil energizing circuit was associated withappropriate circuitry for preventing energization of the coil should theelectronic switch be short circuited.

Because electronic switches are capable of failing by short circuiting,it was necessary to design flame control circuitry to anticipate suchfailures. The end result of such design criteria were somewhat extensiveand complex circuits frequently including large numbers of relativelyexpensive components.

Some of these circuits have been extremely effective and reliable andhave provided functions not previously performable by mechanicalcontrols. One such system is disclosed in copending application, Ser.No. 780,650, filed Dec. 3, 1968, and now U.S. Pat. No. 3,554,680entitled Burner Control System and assigned to the assignee of thisinvention. While effective and reliable, these noted systems have beenlarge and relatively expensive which has limited wide commercialacceptance.

SUMMARY OF THE-INVENTION The present invention provides a new andimproved burner control system which is simple in construction andoperation, reliable, relatively easily manufactured and relativelyinexpensive. The new system utilizes electronic components forcontrolling operation of a solenoid coil, or relay coil, of a fuelsupply means. The system positively prevents energization of the coilshould any components of the system malfunction.

In a preferred embodiment of the invention, a solenoid coil, or relaycoil which controls operation of a fuel valve is connected across an ACpower supply through a voltage doubler arrangement and an electronicswitch. The fuel valve is normallyclosed and the coil is operativelyenergized to open the fuel valve only in response to a predeterminedvoltage applied across the coil. This predetermined voltage is greaterthan the peak power supply voltage. Hence, if the voltage doubler is forany reason ineffective, the coil cannot open the fuel valve even thoughthe coil may continue to be supplied with current from the power supply.

The electronic switch is rendered conductive only when combustion at theburner is sensed, or during a trial for ignition period. The electronicswitch is preferably an SCR, or equivalent, and must function properlyin order for the coil to be operatively energized. The SCR is orientedso that when it is rendered conductive, it permits energization of thecoil only during alternate half cycles of the power supply.

The voltage doubler developes the predetermined voltage during halfcycles in which the SCR blocks conduction through the coil. This voltageis applied to the coil during the half cycles when the SCR conducts tooperatively energize the coil.

A flywheel" capacitor is connected across the coil to provide energy formaintaining the coil operatively energized during alternate half cycleswhen the SCR is nonconductive. Should the SCR become short circuited,the voltage doubler arrangement is rendered ineffective to operativelyenergize the coil because the flywheel capacitor prevents establishmentof the predetermined voltage across the coil. This prevents the fuelvalve from being opened even if the SCR shorts out.

This control circuit is hence quite simple in construction, isconstructed of few, relatively inexpensive components, and yet is notsubject to failing in an unsafe manner.

The new system always fails safely since: l when any component of thevoltage doubler fails, the predetermined voltage cannot be appliedacross the coil; (2) when the flywheel capacitor is short circuited oris opened, the coil does not have sufficient power supplied to it toopen the valve; (3) when the SCR is short circuited the coil cannot beoperatively energized.

In summary, when failure of any component in the circuit occurs, themaximum voltage across the coil is the peak voltage of the power supply.Since this voltage is insufficient to operatively energize the coil, thefailure does not result in an unsafe condition.

A principal object of the present invention is the provision of a newand improved burner control system in which a fuel supply means iscontrolled by a simple, inexpensive, and extremely reliable controlcircuit. I

Other objects and advantages of the present invention will becomeapparent from a consideration of the following detailed descriptionthereof made with reference to the accompanying drawings which form apart of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a schematic burnercontrol system embodying the present invention; and,

FIG. 2 is a circuit diagram of a-portion of the system of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, a burnercontrol system is illustrated which includes a burner 11; a fuel supplysystem 12 for the burner including a gasline 13 and fuel valve Vforopening and closing the line 13; and control circuitry 14 governingoperation of the fuel supply system 12.

The fuel supply system 12 includes a solenoid coil for operating thevalve V from its closed position to its open position. The valve V isnormally closed and is opened only when the solenoid is energized. Thesolenoid 20 is connected across terminals 21, 22 of a suitableelectrical power source through a control switch 23. When the switch 23is closed, the solenoid 20 is energized to open the valve V. When theswitch 23 is open, the solenoid 20 is .deenergized closing the valve V.The control circuitry 14 controls the operation of the switch 23.

The circuitry. 14 is connected to the alternating current power sourcethrough a control switch 24, which is preferably thermostaticallyoperated, and a power supply 25. When the switch 24 is closed, thecircuitry 14 normally functions to close the switch 23 thus opening thefuel valve. When the switch 24 is open, the circuitry 14 closes the fuelvalve.

Referring now to FIG. 2, the power supply 25 is shown as a conventionalstepdown transformer having a primary winding 250 connected across analternating current single phase power source through the switch 24, anda secondary winding 25b having end taps connected to conductors or linesL1, L2, respectively. The conductor L2 is grounded. For the purposes ofthis description, half cycles of the power supply during which thevoltage level at the conductor L2 is positive with respect to theconductor L1 are referred to as positive half cycles. Half cycles of thepower supply producing opposite polarities of the conductors arereferred to as negative half cycles.

The circuitry 14 includes a trial for ignition circuit 26, a flamesensing circuit 27, and a fuel valve operating circuit 28 (FIG. 2).These circuits are all connected across the conductors L1, L2 forenergization from the secondary winding 25!). When the switch 24 closes,the valve operating circuit 28 opens the fuel valve for a trial forignition" period. The extent of this trial period is controlled by thetrial for ignition circuitry 26. During the trial period, the fuel valveis opened to enable a flame to be established at the burner.

If the fuel ignites, the flame-sensing circuit 27 conditions thecircuitry 28 to maintain the fuel valve open so long as flame is presentat the burner. When the flame is extinguished, or the control switch 24opens, or the circuitry 28 malfunctions, the fuel valve closes toterminate fuel flow to the burner.

The valve operating circuitry 28 (FIG. 2) includes a relay circuit 30connected across a voltage doubler 31 through an electronic switch 32.

The electronic switch 32 is preferably an S0 which is connected betweenthe relay circuit 30 and the conductor L2. The SCR is poled to blockconduction through the relay circuit 30 from the conductor L2 to theconductor L1 during positive half cycles of the power supply. The SCRincludes a gate electrode 32a which is connected in a circuit with thetrial for ignition circuit 26 and with the flame-sensing circuit 27. Thetrial for ignition and the flame-sensing circuits may be of any suitableconstruction and are not illustrated in detail; an example of suchcircuitry is shown in the aforementioned copending application, Ser. No.780,650.

The flame-sensing unit 27 and the trial for ignition circuitry bothfunction to produce 60 cycle pulses during negative half cycles of thepower supply 25, under conditions described presently. When either ofthese units produce pulses, the pulses are transmitted to the gateelectrode of the SCR. The SCR is then rendered conductive to complete acircuit through the relay circuit 30 for opening the fuel valve.

The trial for ignition circuitry 26 produces 60 cycle pulses for alimited period after the switch 24 closes. The unit 26 thus provides alimited period of time during which a flame may be established at theburner 11.

For the purposes of this description, the flame-sensing unit 27 isillustrated as utilizing the rectifying effect of a flame to electricalcurrent. This unit produces 60 cycle pulses in response to the presenceof flame between a flame-detecting electrode 27a and the electricallygrounded burner 11. If flame is not established during the trial forignition period, the pulses produced by the trial for ignition circuitryterminate and the flame sensing unit fails to produce pulses. The SCR isaccordingly prevented from further conduction and the relay circuit isdeenergized, closing the fuel valve. When flame is established duringthe trial for ignition period, the flamesensing unit continues toproduce pulses after the period and the SCR maintains the relay circuitconductive during negative half cycles so that the fuel valve is opened.

The relay circuit 30 includes a relay coil 55 and a resistor 5 connectedin series with the coil 55. A capacitor 60 is connected in parallel withthe relay coil 55 and the resistor 56 between junctions 61, 62.

The relay coil 55 cooperates with an armature 65 which is linked to theswitch 23. When the coil 55 is operatively energized to pull in thearmature 65, the switch 23 closes, thus opening the fuel valve. In orderto pull in the armature 65 the coil 55 must be provided with apredetermined minimum applied voltage. When this predetermined voltageis applied across the coil, it is operatively energized to effectopening of the gas valve. If, on the other hand, current flows throughthe coil 55 but the applied voltage is less than the predeterminedvoltage, the relay, even though conducting, is not operativelyenergized; that is to say, the armature 65 cannot be pulled in.

The coil 55 is bistable in that once the coil is operatively energizedto pull in the armature, the armature remains pulled in even though thevoltage applied across the coil diminishes below the predeterminedlevel. The resistor 56 has an impedance which is selected so that therelay coil 55 can only be operatively energized by a voltage appliedacross the junctions 61, 62 which is substantially greater than the peakpower supply voltage; i.e., the peak voltage across the end taps of thesecondary winding 25b. in other words, the coil 55 is only operativelyenergized in response to application of a predetermined voltage acrossit which is greater than the maximum power supply voltage which can beapplied to it.

When the voltage applied across the coil 55 and resistor 56 is less thanthis predetermined voltage, the coil 55, even though conducting, is notoperatively energized and the fuel valve cannot open.

The capacitor 60 is a flywheel capacitor which maintains I the coiloperatively energized during positive half cycles of the power supply.The capacitor 60 is charged when the relay coil is operatively energizedthrough the SCR during negative half cycles. During positive half cyclesthe capacitor discharges through the coil to provide a sustainingcurrent to maintain the armature pulled in.

The voltage doubler 31 developes voltages which are effective tooperatively energize the relay coil 55 through the SCR 32. The voltagedoubler 31 includes a diode and a capacitor 71 which are connected toproduce voltages having peak amplitudes of about twice the peak voltageamplitudes of the power supply during negative half cycles. The diode 70is connected across the conductors L1, L2 between junctions 73, 74, andis poled to block current flow between the junctions during positivehalf cycles. The capacitor 71 is connected in the conductor L1 betweenthe junction 73 and the secondary 251;. During positive half cycles thecapacitor 71 charges through the diode so that at the end of eachpositive half cycle the plate 710 is positive with respect to the plate71b by an amount substantially equal to the peak voltage of the powersupply. During the succeeding negative half cycle the voltage level atthe plate 71b rises and since the charge across a capacitor cannotchange instantaneously, the voltage level from theplate 71a to theconductor L2 increases accordingly,. This level rises to about twice thepeak voltage of the power supply.

The doubled voltage output of the doubler 31 is applied across the relaycircuit 30 through the SCR 32. Hence, when the SCR is renderedconductive during negative half cycles, the voltage applied to the coil55 is more than sufficient to operatively energize the coil. Thecapacitor 60 connected in parallel with the coil 55 and resistor 56 ischarged to about the doubled voltage level since the voltage drop acrossthe SCR is minimal. Hence, the capacitor 60 provides the predeterminedvoltage to the coil 55 as the capacitor discharges during positive halfcycles and the armature remains pulled in.

FAILSAFE OPERATlON From the foregoing, it is apparent that the relaycoil 55 is operatively energizable to open the fuel valve only when theSCR 32 isconductive and then only when a predetermined voltage, which issubstantially greater than the peak voltage of the power supply, isapplied across the relay circuit 30.

If any component of the circuitry 28 fails, the fuel valve cannot beopened and hence the circuit is failsafe. This is true even though therelay coil 55 might be conductive and the peak power supply voltageapplied across the relay circuit 30.

If either the capacitor 71 or the diode 70 fails, the voltage doublerwill not function and the relay coil 55 cannot be provided with anadequately high voltage to effect opening of the fuel valve. if theplates of the capacitor 71 are short circuited, that capacitor cannot becharged and the predetermined voltage will not be developed. If thecapacitor 71 fails to open the conductor Ll, the relay coil isdisconnected from the power supply. Shorting of the diode 70 causes theboth half cycles of the power supply current to appear across thecapacitor 71 and prevents the doubler 31 from generating the requisitedoubled voltage for operatively energizing the relay coil. If the diode70 fails open, the capacitor 71 cannot be charged.

If the plates of the capacitor 60 are short circuited, the capacitor 60can no longer function as a flywheel" for the relay coil and the valve Vwill not open. The shorted capacitor also starves the coil 55 forcurrent during negative half cycles. if the capacitor 60 fails open, itsflywheeling function cannot be performed.

if the SCR 32 is shorted, it will no longer block current flow duringpositive half cycles. This charges the capacitor 60 to oppose operationof the voltage doubler and the predetermined voltage is not appliedacross the relay coil 55 during negative half cycles. If the SCR isopen, the coil 55 is disconnected from the power supply.

The resistor 56 will only fail so as to open the circuit through thecoil 55, and such failure precludes opening of the fuel valve.

While the relay circuit 30 is illustrated and described as including arelay coil 55, it should be apparent that the disclosed relay circuitrycould be modified to provide a valve operating solenoid coil in place ofthe relay coil 55.

Although only a single embodiment of the invention has been illustratedand described in considerable detail, it will be appreciated thatvarious modifications, adaptations and uses of this invention willbecome apparent to those skilled in this art and it is the intention tocover hereby all such modifications, adaptations and uses.

What is claimed is:

1. In a burner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprisa. an alternating currentelectrical power supply;

b. a fuel supply controlling circuit comprising a voltage responsiveelement for effecting operation of said fuel supply means and having afirst condition in which the fuel supply means prevents the supply offuel to said burner and a second condition wherein said fuel supplymeans enables the supply of fuel to said burner;

c. said element operable to said second condition only in response toapplication of a predetermined voltage thereacross which is greater thanthe maximum peak voltage which can be applied thereacross directly fromsaid power supply;

d. an electronic switch means connected in a series circuit with saidelement for completing and interrupting a circuit through said elementin response to a detected condition; and,

e. a voltage increasing circuit connected to said power supply andhaving an output connected across said element and said electronicswitch means, said voltage increasing circuit effective to produce anoutput voltage across said element which is in excess of thepredetermined voltage;

. said electronic means properly operating to complete a circuit acrosssaid output of said voltage increasing circuit through said voltageresponsive element only during first alternate half cycles of said powersupply in response to said detected condition and blocking conductionduring all second alternate power supply half cycles and during firstalternate power supply half cycles when said condition is not detected;and,

g. means connected to said voltage increasing circuit and saidelectronic switch means for rendering said voltage increasing circuitineffective to produce said predetermined voltage when said electronicswitch means is improperly conductive during second alternate powersupply half cycles whereby said voltage responsive element isimmediately operated to and maintained in said first conditionregardless of detection of said condition.

2. A system as claimed in claim l and further including means fordetecting combustion at said burner, said detecting means effective tooperate said switch to complete said circuit through said element.

3. In combustion apparatus including burner control circuitry foropening a normally closed fuel valve in response to combustion at saidburner, said circuitry comprising:

a. an electrical power supply;

b. a valve operating circuit comprising an operating element foreffecting opening of said valve, said valve operating circuit effectingopening of said valve only in response to a predetermined appliedvoltage across said valve operating circuit which is of greatermagnitude than the maximum voltage which can be applied thereacrossdirectly from said power supply;

c. circuit means connected to said power supply and having an outputconnected to said valve operating circuit for applying at least saidpredetermined voltage across said valve operating circuit;

d. combustion responsive switching means connected in series with saidelement, said switching means rendered conductive in response todetection of combustion whereby said valve is maintained open byoperation of said valve operating circuit, said switching meansremaining nonconductive at all other times during proper operationthereof; and,

e. means cooperating with said switching means and said circuit meansfor disabling said circuit means from providing said predeterminedvoltage when said switching means is improperly conductive whereby saidoperating element is immediately rendered ineffective to open said valveeven though said valve operating circuit is conductive.

4. A system as claimed in claim 3 wherein said power supply producesalternating current and said circuit means comprises a voltage doublerconnected to said power supply and effective to provide saidpredetermined voltage to said element.

5. In a burner control system having a burner, and fuel supply meansoperable to permit and prevent the supply of fuel to said burner,circuitry for controlling operation of said fuel supply meanscomprising:

a. an electric power supply;

b. a voltage responsive coil element having a first condition in whichsaid fuel supply means prevents the supply of fuel to said burner and asecond condition for enabling said fuel supply means to permit fuel flowto said burner;

. said voltage-responsive coil element operated to said second conditiononly in response to a predetermined applied voltage greater than thepower supply voltage;

. circuit means for applying said predetermined voltage to supply meansfor controlling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising:

an alternating current electrical power supply;

a fuel supply controlling circuit comprising a voltageresponsive elementfor effecting operation of said fuel supply means and having a firstcondition in which the fuel supply means prevents the supply of fuel tosaid burner and a second condition wherein said fuel supply meansenables the supply of fuel to said burner;

said element operable to said second condition only in response to apredetermined voltage thereacross which is greater than the maximumvoltage which could be applied thereacross directly from said powersupply, said voltage-responsive element being, at all other times, insaid first condition wherein said fuel supply means prevents supply offuel to said burner;

an electronic switch means connected in a circuit with said element forcompleting and interrupting a circuit through said element, said switchmeans capable of conduction only during first alternate half cycles ofsaid power supply when operating properly;

a voltage increasing circuit connected to said power supply and acrosssaid element and said switch means, and voltage increasing circuiteffective to produce an output voltage across said element which is inexcess of the predetermined voltage;

combustion detecting means at said burner, said detecting meanseffective to operate said switch means to complete said circuit throughsaid element; and,

means cooperating with said switch means and said voltage increasingmeans for rendering said voltage increasing means ineffective to providesaid predetermined voltage across said element when said switch means isimproperly conductive during second alternate power supply half cycles.

In a burner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising:

an alternating current electrical power supply;

a fuel supply controlling circuit comprising a voltageresponsive elementfor effecting operation of said fuel supply means and having a firstcondition in which the fuel supply means prevents the supply of fuel tosaid burner and a second condition wherein said fuel supply meansenables the supply of fuel to said burner;

said element operable to said second condition only in response to apredetermined voltage applied across said fuel supply controllingcircuit which is greater than the maximum voltage which can be appliedthereacross directly from said power supply;

an electronic switch means connected in a circuit with said element forcompleting and interrupting a circuit through said element, said switchmeans being capable of completing said circuit during first alternatehalf cycles of v said power supply and blocking current to said elementduring second alternate half cycles when properly operata voltageincreasing circuit connected tosaid power supply and across said fuelsupply controlling circuit and said switch means, said voltageincreasing circuit effective to produce an output voltage across saidfuel supply controlling circuit which is in excess of the predeterminedvoltage; and,

f. means connecting said switch means and said voltage increasingcircuit for disabling said voltage increasing circuit when said switchmeans is improperly conductive during second alternate power supply halfcycles, said voltage increasing circuit thereby rendered ineffective toproduce said predetermined voltage to prevent fuel supply to the burnerso long as said switch means is improperly conductive.

8. In a burner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising:

a. an alternating current electrical power supply;

b. a fuel supply controlling circuit comprising a voltageresponsiveelement for effecting operation of said fuel supply means and having afirst condition in which the fuel supply means prevents the supply offuel to said burner and a second condition wherein said fuel supplymeans enables the supply of fuel to said burner;

c. said element operable to said second condition only in response to apredetermined voltage thereacross which is greater than the maximumvoltage which can be applied thereacross directly from said powersupply;

d. an electronic switch means having a control electrode, saidelectronic switch capable of conduction only during first alternate halfcycles of said power supply and blocking conduction during secondalternate power supply half cycles;

. a voltage doubling circuit connected to said power supply and acrosssaid fuel supply controlling circuit and said switch means, saiddoubling circuit effective to produce an output voltage across saidelement which is in excess of the predetermined voltage;

f. combustion responsive circuitry for providing signals to said controlelectrode for rendering said switch conductive during said firstalternate half cycles, said combustion responsive circuitry effective toprovide said signals only in response to combustion at said burner; and,

means cooperating with said electronic switch for preventing saidvoltage doubler circuit from producing said predetermined voltage whensaid electronic switch is improperly conductive during second alternatepower supply half cycles, said voltage responsive element thereuponbeing operated to its first condition so that fuel supply to said burneris prevented.

9. ln a burner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising:

a. an alternating current electrical power supply;

b. a fuel supply controlling circuit including a voltageresponsive coilfor effecting operation of said fuel supply means and having a firstcondition in which the fuel supply means prevents the supply of fuel tosaid burner and a second condition wherein said fuel supply meansenables the supply of fuel to said burner;

. said coil operable to said second condition only in response to apredetermined voltage thereacross which is greater than the maximumvoltage which can be applied thereacross by said power supply;

d. an electronic switch means connected in a circuit with said coil forcompleting and interrupting a circuit through said coil, said switchmeans completing said circuit during first alternate half cycles of saidpower supply and blocking current to said coil during second alternatehalf cycles.

e. a voltage doubling circuit connected to said power supply and acrosssaid coil and said switch means, said doubling circuit effective toproduce an output voltage across said coil which is in excess of thepredetermined voltage; and,

f. a capacitor connected in parallel with said coil, said capacitorcharging to at least said predetermined voltage through said switchmeans when said coil is in the second condition and discharging throughsaid coil during said second alternate half cycles of said power supplyto maintain said coil in said second condition when said switch meansblocks conduction.

10. The system claimed in claim 1 wherein said means for rendering saidvoltage increasing circuit comprises a capacitor connected in parallelwith said voltage-responsive element.

11. The system claimed in claim 10 wherein said voltage increasingcircuit comprises a voltage doubling circuit including a capacitorconnected in series with said fuel supply controlling circuit and adiode connected in parallel with said fuel supply controlling circuit,said doubler effective to provide a pulsating DC voltage to said fuelsupply controlling circuit which is greater than the peak power supplyvoltage.

12. The system claimed in claim 10 wherein said voltageresponsiveelement comprises a coil and said fuel controlling circuit furthercomprises a resistor connected in series with said coil to control thevoltage drop across said coil when said electronic switch meansconducts.

1. In a burner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising: a. an alternating currentelectrical power supply; b. a fuel supply controlling circuit comprisinga voltage responsive element for effecting operation of said fuel supplymeans and having a first condition in which the fuel supply meansprevents the supply of fuel to said burner and a second conditionwherein said fuel supply means enables the supply of fuel to saidburner; c. said element operable to said second condition only inresponse to application of a predetermined voltage thereacross which isgreater than the maximum peak voltage which can be applied thereacrossdirectly from said power supply; d. an electronic switch means connectedin a series circuit with said element for completing and interrupting acircuit through said element in response to a detected condition; and,e. a voltage increasing circuit connected to said power supply andhaving an output connected across said element and said electronicswitch means, said voltage increasing circuit effective to produce anoutput voltage across said element which is in excess of thepredetermined voltage; f. said electronic means properly operating tocomplete a circuit across said output of said voltage increasing circuitthrough said voltage responsive element only during first alternate halfcycles of said power supply in response to said detected condition andblocking conduction during all second alternate power supply half cyclesand during first alternate power supply half cycles when said conditionis not detected; and, g. means connected to said voltage increasingcircuit and said electronic switch means for rendering said voltageincreasing circuit ineffective to produce said predetermined voltagewhen said electronic switch means is improperly conductive during secondalternate power supply half cycles whereby said voltage responsiveelement is immediately operated to and maintained in said firstcondition regardless of detection of said condition.
 2. A system asclaimed in claim 1 and further including means for detecting combustionat said burner, said detecting means effective to operate said switch tocomplete said circuit through said element.
 3. In combustion apparatusincluding burner control circuitry for opening a normally closed fuelvalve in response to combustion at said burner, said circuitrycomprising: a. an electrical power supply; b. a valve operating circuitcomprising an operating element for effecting opening of said valve,said valve operating circuit effecting opening of said valve only inresponse to a predetermined applied voltage across said valve operatingcircuit which is of greater magnitude than the maximum voltage which canbe applied thereacross directly from said power supply; c. circuit meansconnected to said power supply and having an output connected to saidvalve operating circuit for applying at least said predetermined voltageacRoss said valve operating circuit; d. combustion responsive switchingmeans connected in series with said element, said switching meansrendered conductive in response to detection of combustion whereby saidvalve is maintained open by operation of said valve operating circuit,said switching means remaining nonconductive at all other times duringproper operation thereof; and, e. means cooperating with said switchingmeans and said circuit means for disabling said circuit means fromproviding said predetermined voltage when said switching means isimproperly conductive whereby said operating element is immediatelyrendered ineffective to open said valve even though said valve operatingcircuit is conductive.
 4. A system as claimed in claim 3 wherein saidpower supply produces alternating current and said circuit meanscomprises a voltage doubler connected to said power supply and effectiveto provide said predetermined voltage to said element.
 5. In a burnercontrol system having a burner, and fuel supply means operable to permitand prevent the supply of fuel to said burner, circuitry for controllingoperation of said fuel supply means comprising: a. an electric powersupply; b. a voltage responsive coil element having a first condition inwhich said fuel supply means prevents the supply of fuel to said burnerand a second condition for enabling said fuel supply means to permitfuel flow to said burner; c. said voltage-responsive coil elementoperated to said second condition only in response to a predeterminedapplied voltage greater than the power supply voltage; d. circuit meansfor applying said predetermined voltage to said coil element in responseto a sensed condition, said circuit means comprising an electronicswitch connected between said power supply and said coil for blockingcurrent flow in one direction and rendered conductive to enable currentflow in said coil in an opposite direction; and, e. a capacitorconnected in parallel with said coil said capacitor charging when saidswitch is conductive and discharging when said switch blocks currentflow to said element.
 6. In a burner control system having a burner, andfuel supply means for controlling the supply of fuel to said burner,electrical circuitry controlling the fuel supply means comprising: a. analternating current electrical power supply; b. a fuel supplycontrolling circuit comprising a voltage-responsive element foreffecting operation of said fuel supply means and having a firstcondition in which the fuel supply means prevents the supply of fuel tosaid burner and a second condition wherein said fuel supply meansenables the supply of fuel to said burner; c. said element operable tosaid second condition only in response to a predetermined voltagethereacross which is greater than the maximum voltage which could beapplied thereacross directly from said power supply, saidvoltage-responsive element being, at all other times, in said firstcondition wherein said fuel supply means prevents supply of fuel to saidburner; d. an electronic switch means connected in a circuit with saidelement for completing and interrupting a circuit through said element,said switch means capable of conduction only during first alternate halfcycles of said power supply when operating properly; e. a voltageincreasing circuit connected to said power supply and across saidelement and said switch means, and voltage increasing circuit effectiveto produce an output voltage across said element which is in excess ofthe predetermined voltage; f. combustion detecting means at said burner,said detecting means effective to operate said switch means to completesaid circuit through said element; and, g. means cooperating with saidswitch means and said voltage increasing means for rendering saidvoltage increasing means ineffective to provide said predeterminedvoltage across said element when said switch means is improperlyconduCtive during second alternate power supply half cycles.
 7. In aburner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising: a. an alternating currentelectrical power supply; b. a fuel supply controlling circuit comprisinga voltage-responsive element for effecting operation of said fuel supplymeans and having a first condition in which the fuel supply meansprevents the supply of fuel to said burner and a second conditionwherein said fuel supply means enables the supply of fuel to saidburner; c. said element operable to said second condition only inresponse to a predetermined voltage applied across said fuel supplycontrolling circuit which is greater than the maximum voltage which canbe applied thereacross directly from said power supply; d. an electronicswitch means connected in a circuit with said element for completing andinterrupting a circuit through said element, said switch means beingcapable of completing said circuit during first alternate half cycles ofsaid power supply and blocking current to said element during secondalternate half cycles when properly operating; e. a voltage increasingcircuit connected to said power supply and across said fuel supplycontrolling circuit and said switch means, said voltage increasingcircuit effective to produce an output voltage across said fuel supplycontrolling circuit which is in excess of the predetermined voltage;and, f. means connecting said switch means and said voltage increasingcircuit for disabling said voltage increasing circuit when said switchmeans is improperly conductive during second alternate power supply halfcycles, said voltage increasing circuit thereby rendered ineffective toproduce said predetermined voltage to prevent fuel supply to the burnerso long as said switch means is improperly conductive.
 8. In a burnercontrol system having a burner, and fuel supply means for controllingthe supply of fuel to said burner, electrical circuitry controlling thefuel supply means comprising: a. an alternating current electrical powersupply; b. a fuel supply controlling circuit comprising avoltage-responsive element for effecting operation of said fuel supplymeans and having a first condition in which the fuel supply meansprevents the supply of fuel to said burner and a second conditionwherein said fuel supply means enables the supply of fuel to saidburner; c. said element operable to said second condition only inresponse to a predetermined voltage thereacross which is greater thanthe maximum voltage which can be applied thereacross directly from saidpower supply; d. an electronic switch means having a control electrode,said electronic switch capable of conduction only during first alternatehalf cycles of said power supply and blocking conduction during secondalternate power supply half cycles; e. a voltage doubling circuitconnected to said power supply and across said fuel supply controllingcircuit and said switch means, said doubling circuit effective toproduce an output voltage across said element which is in excess of thepredetermined voltage; f. combustion responsive circuitry for providingsignals to said control electrode for rendering said switch conductiveduring said first alternate half cycles, said combustion responsivecircuitry effective to provide said signals only in response tocombustion at said burner; and, g. means cooperating with saidelectronic switch for preventing said voltage doubler circuit fromproducing said predetermined voltage when said electronic switch isimproperly conductive during second alternate power supply half cycles,said voltage responsive element thereupon being operated to its firstcondition so that fuel supply to said burner is prevented.
 9. In aburner control system having a burner, and fuel supply means forcontrolling the supply of fuel to said burner, electrical circuitrycontrolling the fuel supply means comprising: a. an alternating currentelectrical power supply; b. a fuel supply controlling circuit includinga voltage-responsive coil for effecting operation of said fuel supplymeans and having a first condition in which the fuel supply meansprevents the supply of fuel to said burner and a second conditionwherein said fuel supply means enables the supply of fuel to saidburner; c. said coil operable to said second condition only in responseto a predetermined voltage thereacross which is greater than the maximumvoltage which can be applied thereacross by said power supply; d. anelectronic switch means connected in a circuit with said coil forcompleting and interrupting a circuit through said coil, said switchmeans completing said circuit during first alternate half cycles of saidpower supply and blocking current to said coil during second alternatehalf cycles. e. a voltage doubling circuit connected to said powersupply and across said coil and said switch means, said doubling circuiteffective to produce an output voltage across said coil which is inexcess of the predetermined voltage; and, f. a capacitor connected inparallel with said coil, said capacitor charging to at least saidpredetermined voltage through said switch means when said coil is in thesecond condition and discharging through said coil during said secondalternate half cycles of said power supply to maintain said coil in saidsecond condition when said switch means blocks conduction.
 10. Thesystem claimed in claim 1 wherein said means for rendering said voltageincreasing circuit comprises a capacitor connected in parallel with saidvoltage-responsive element.
 11. The system claimed in claim 10 whereinsaid voltage increasing circuit comprises a voltage doubling circuitincluding a capacitor connected in series with said fuel supplycontrolling circuit and a diode connected in parallel with said fuelsupply controlling circuit, said doubler effective to provide apulsating DC voltage to said fuel supply controlling circuit which isgreater than the peak power supply voltage.
 12. The system claimed inclaim 10 wherein said voltage-responsive element comprises a coil andsaid fuel controlling circuit further comprises a resistor connected inseries with said coil to control the voltage drop across said coil whensaid electronic switch means conducts.